This invention relates to an igniter for an internal combustion engine. More particularly, it relates to an igniter having power transistors which are protected from excessive current.
An engine igniter is generally equipped with power transistors which control the current flowing through the primary winding of an ignition coil. As substantially all the primary winding current flows through the power transistors, it is necessary to protect the power transistors from damage due to excessive current. In a typical arrangement, a current sensing resistor is connected in series with each power transistor, and the base current of the power transistor is controlled by a base current control circuit in accordance with the voltage across the current sensing resistor. The base current control circuit generally includes a potentiometer for adjusting its operating characteristics.
The above-described arrangement has the disadvantage that it employs as many current sensing resistors and potentiometers as there are power transistors, so in an engine which has two or more cylinders, a large number of components is necessary. Furthermore, the current sensing resistors have a relatively large power consumption, so if they are in the form of thick-film components, they take up much space and increase the size of the igniter. In addition, it is necessary to initially adjust the potentiometer associated with each base current control circuit to a prescribed value. As there are a large number of potentiometers in the igniter, the adjusting process is time-consuming and costly.